\makeatletter
\ifthenelse{\boolean{ABCIbiblabelonmargin}}
{
  \renewcommand{\@biblabel}[1]%
  {\ifthenelse{\equal{#1}{}}{}{{\citenumstyle [#1]\hspace{\biblabelsep}}}}
}
{
  \renewcommand{\@biblabel}[1]%
  {%
    \ifthenelse{\equal{#1}{}}
    {}
    {%
      \def\biblabeltext{{\citenumstyle [#1]\hspace{\biblabelsep}}}%
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      \ifthenelse{\lengthtest{\ABCIauxlen<\minimumbiblabelwidth}}
      {\setlength{\ABCIauxlen}{\minimumbiblabelwidth-\ABCIauxlen}}
      {\setlength{\ABCIauxlen}{0cm}}%
      {\biblabeltext\hspace{\ABCIauxlen}}%   
    }%
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}
\makeatother